1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) generator, and more particularly to a surface acoustic wave generator suitable for use in an optical wave guide type acousto-optical (AO) system.
2. Related Background Art
A requirement for an SAW transducer used as a component of an optical wave guide type AO system such as light deflector, spectrum analyzer or correlator is more and more wide band characteristics. Several methods have been known to manufacture a wideband SAW transducer. Among others, a multiple tilted SAW transducer array (MTS array) having a plurality of interdigital transducers (IDT'S) having different center frequencies arranged at different tilt angles to meet a Bragg's condition for a guided light is simple in construction and may be electrically adjusted after the preparation of the transducer. For example, U.S. Pat. No. 4,455,064 (date of patent: June 19, 1984) shows and describes such an MTS array in FIG. 1 and column 2, line 65 to column 3, line 27.
In order to excite a surface acoustic wave in an optical wave guide by such an MTS array and cause a desired AO effect with a guided wave propagating in the optical wave guide, it is necessary to apply an HF signal having specific frequency and power to each of the IDT's. Such an HF signal is usually generated and amplified by an electric circuit and supplied to the IDT's. Such an electric circuit is called an IDT driver and it is shown and described in U.S. Pat. No. 4,394,060 (date of patent July 19, 1983) in FIG. 1 and column 4, line 13 to column 5line 50.
FIG. 1 shows a prior art surface acoustic wave generator including an IDT driver.
In FIG. 1, numeral 1 denotes an optical wave guide including a LiNbO.sub.3 crystal substrate having Ti diffused on a surface thereof, and numeral 2 denotes a guided light (incident light) which propagates along the optical wave guide. Numerals 3, 4, 5 and 6 denote IDT's having different center frequencies and are arranged at different tilt angles to the guided light 2. Numeral 7 denotes an SAW which is excited from the IDT's and propagates along the optical wave guide 1, numeral 8 denotes a diffraction light generated by the diffraction of the SAW by the incident light 2, and numeral 9 denotes a non-diffraction light.
An HF signal generated by a variable frequency oscillator 11 is amplified by an amplifier 12 to a desired power and it is distributed by a power distributor 13 and supplied to the IDT's 3, 4, 5 and 6 through variable phase shifters 14 and matching circuits 15. The IDT's have different center frequencies and specific frequency band widths. Only one or at most two IDT's of the four IDT's are substantially driven to generate the SAW. In FIG. 1, the drive frequency of the transducer array is such that the SAW is excited from only the IDT 4. The SAW 7 excited by the IDT 4 acts as a diffraction grating which moves relative to the incident light, and a portion of the incident light 2 is Bragg-diffracted to form a diffraction light 8, a portion of which transmits to form a non-diffraction light 9. The diffraction light 8 is separated from the non-diffraction light 9 and used for signal processing and recording.
The variable phase shifter 14 compensates for a phase difference between the diffraction lights by the IDT's at a crossover frequency of the two adjacent IDT's to adjust the phases of the SAW's so that the SAW's from the two IDT's strengthen each other. As shown in the U.S. Pat. No. 4,455,064, the same effect may be attained without the variable phase shifters by appropriately designing the IDT arrays.
In the prior art surface acoustic wave generator, insertion losses of the variable phase shifters and matching circuits may be reduced in order to reduce a power consumption of the driver, and electrode resistances of the IDT's may be reduced and an impedance in a specific pass frequency band may be matched to a signal source for the same purpose.
However, when the electrode resistances of the IDT's are reduced and the impedance in the pass frequency band is matched to the signal source, the IDT's are of high impedance at a frequency other than the pass frequency band and the HF signal supplied to the IDT's is reflected to the signal source without substantial loss. The reflected signal passes through the matching circuits 15 and the variable phase shifters 14. Since those electrical circuits are usually passive reciprocal circuits, if the insertion losses of those circuits are reduced, the HF signals reflected from the IDT's pass without substantial loss and return to the power distributor 13 to interfere with an input signal to the distributor 13 from the amplifier 12.
For example, when the HF signal from the variable frequency generator 11 is near the center frequency of the IDT 4 as shown in FIG. 1, the HF signal is beyond the frequency bands of the IDT's 3, 5 and 6. Thus, the HF signals reflected by those IDT's return to the power distributors 13 and interfere with the input signal to the distributor 13, from the amplifier 12. Since the phase of the HF signal reflected by the IDT and returned to the power distributor 13 depends on the frequency, the frequency characteristic of the HF signal supplied to the IDT 4 includes a large ripple.
FIG. 2 shows a graph of simulation of the ripple generation. In this simulation, the transducer comprises three IDT's, each IDT has five pairs of electrode fingers, the first IDT has a center frequency of 360 MHz, the second IDT has a center frequency of 400 MHz, the third IDT has a center frequency of 440 MHz, the IDT's are matched to a signal source 50 Ohms by inductors, and the IDT arrays are appropriately designed so that the same effect is attained without the variable phase shifter.
In FIG. 2, #1, #2 and #3 represent the first, second and third IDT's, respectively. Broken lines show results obtained when three IDT's are individually driven, and solid lines show results obtained when the three IDT's are driven in parallel. It is seen from FIG. 2 that a large ripple is created in the frequency band due to the reflected HF signals from other IDT's when the driver configuration shown in FIG. 1 is used.
When the frequency characteristic of the HF signal applied to the IDT's includes such a ripple, intensity of the SAW which serves to diffract the guided light 2 of FIG. 1 varies with the frequency. As a result, the intensity of the diffraction light 8 varies with the frequency. Accordingly, in the prior art surface acoustic wave generator shown in FIG. 1, if the loss of the circuit is to be reduced to reduce the power consumption, the intensity of the diffraction light 8 is not uniform for the change of the frequency of the HF signal used to excite the SAW 7.